The immune system of mammals must possess the capability to react to a very large number of foreign antigens. Lymphocytes constitute a central element of the immune system because they can recognize antigens and effect a specific, adaptive immune response. Lymphocytes can be divided into two general classes of cells, B-lymphocytes, which are capable of expressing antibodies, and T lymphocytes that can be sub-divided into CD4+ helper T cells and CD8+ cytotoxic T cells. Both of these sub-groups of T lymphocytes are capable of recognizing antigens associated with surface proteins known as the major histocompatibility complex (MHC). The recognition of the MHC occurs throughout the T cell receptor (TCR), a protein complex that is anchored in the cytoplasmic membrane of T cells. The CD8+ T cell receptor exclusively mediates interactions between MHC class I antigens and cytotoxic T cells; the CD4+ T cell receptor exclusively mediates interactions between MHC class II antigens and helper T cells.
The triggering of an immune response does not exclusively progress from T cells alone, but rather, through the interaction of T cells with so-called antigen presenting cells (APCs, also known as accessory cells) and their surface markers (for example MHC II).
These accessory cells can be sub-divided into “simple” APCs whose function is to present antigens and “professional” APCs that, beside from presenting antigens, also have an accessory function in stimulating lymphocytes. APCs themselves do not have antigen specificity but serve as “nature's adjuvant” by presenting antigens to T cells. Aside from mononuclear phagocytes, dendritic cells (DC) are members of the APC type. In fact, DCs are the most potent APC known today and they are the only APC that are also able to stimulate naive T cells and are therefore called “natures adjuvants”. As a result of their different characteristics and function, two types of dendritic cells have been classified to date:
follicular dendritic cells (also known as lymphoid-related DCs) that are present in the lymph nodes, spleen and mucosa-associated lymph tissues and interdigitating dendritic cells (also known as myeloid derived DCs) that are found in the interstitial space of most organs, in T cell rich zones of the lymph nodes and spleen and are distributed throughout the skin where they are known as Langerhans cells.
Immature dendritic cells, i.e. DCs that are not fully capable of stimulating T cells, have the function of taking up antigens and processing them into MHC-peptide complexes. Stimuli such as TNF-alpha (tumor necrosis factor) and CD40L induce the maturation of dendritic cells and lead to a massive de novo synthesis of MHC class I and MHC class II molecules and to a migration of the DC, for example, from the interstitial space of the internal organs through the blood into the lymph nodes of the spleen and liver. Moreover, increased expression of co-stimulator molecules (for example, CD80, CD86) and adhesion molecules (for example, LFA3) occurs during the migration phase into the secondary lymphoid tissues. Mature DC stimulate T lymphocytes upon arrival in the T cell rich regions of the secondary lymphoid tissue by presenting peptide antigens within the context of MHC class I or MHC class II to these T cells. Depending on the conditions, DCs can stimulate the activation of a variety of T cells which, in turn, can bring about a differential response of the immune system. For example, as mentioned above, DCs that express MHC class I can cause cytotoxic T cells to proliferate and DCs that express MHC class II can interact with helper T cells. In the presence of mature DCs and the IL-12 that they produce, these T cells differentiate into Th1 cells that produce interferon-gamma.
Interferon-gamma and IL-12 serve together to promote T-killer cells. In the presence of IL-4, DCs induce T cells to differentiate into Th2 cells which secrete IL-5 and IL-4 that in turn activates eosinophils and assist B cells to produce antibodies (Banchereau, J. and Steinman, R. M. (1998) Nature 392: 245-252).
DCs can also induce a so-called mixed leukocyte reaction (MLR) in vitro, a model for allogenic T cell activation and graft rejection.
A typical feature of these MLR-assays is the formation of large DC-T cell-clusters. Addition of hCD83ext at day 1 strongly inhibited the typical cell cluster formation of DC and proliferating T cells (Lechmann, M. et al. (2001) J. Exp. Med. 194: 1813-1821).
Mature DC characteristically express, amongst others (e.g. MHC I and II, CD80/86, CD40) the marker molecule CD83 on their cell surface (Zhou, L.-J. and Tedder, T. F. (1995) J. Immunology, vol. 154: 3821-3835). This is one of the best markers for mature DC known today.
CD83, a molecule from the Ig superfamily of proteins, is a single chain, 43 kDa glycoprotein consisting of 205 amino acids (SEQ ID NO:2) in its immature form. The first 19 amino acids represent the signal peptide of CD83 and they are lost upon insertion of the protein into the membrane, leaving a 186 amino acid membrane spanning protein. The mature CD83 has an extracellular domain formed by amino acids 20 to 144 (SEQ ID NO:2), a transmembrane domain comprising amino acids 145 to 166 (SEQ ID NO:2), and cytoplasmic domain formed by amino acids 167 to 205 (SEQ ID NO:2). The extracellular domain has as structural feature a single Ig-like (V-type) domain, and is expressed very strongly on the cell surface of mature DC. The extracellular domain of the CD83 protein differs from the typical Ig-like domain in that it is encoded by at least two exons: one exon only codes for a half of the Ig-like domain, whereas the other exon encodes the membrane spanning domain (see Zhou, L.-J., Schwarting, R., Smith, H. M. and Tedder, T. F. (1999) J. Immunology, vol. 149: 735-742). The cDNA encoding human CD83 contains a 618 bp open reading frame (SEQ ID NO:1, see Genbank ID: Z11697 and Zhou, L.-J. et al, supra (1995)).
While the precise function of CD83 remains to be determined, it has been demonstrated that inhibition of CD83 cell surface expression on mature DC by interference with nuclear export of CD83 mRNA leads to a clear reduction in the capacity of these cells to stimulate T cells. (Kruse, M. et al. (2000) J. Exp. Med. 191: 1581-1589). Thus, CD83 appears to be required for DC function. Furthermore it was found that when a soluble form of CD83 was administerd to cells, the amount of CD83 expressed by the cells was reduced (mature dendritic cells) or the cells did not start to produce CD83 (immature dendritic cells). Since immature dendritic cells have no CD83 in/on their membrane, this observation lead to the conclusion, that soluble CD83 must interact with another cell (membrane) protein than CD83, i.e. a heterophilic interaction is suspected to occur between the soluble CD83 and an unidentified ligand (Lechmann, M. et al. (Dez. 17, 2001) J. Exp. Med. 194: 1813-1821 and (June 2002) Trends in Immunology, Vol. 23(6):273-275). Evidence for the occurrence of soluble CD83 in vivo also exist. Soluble CD83 has been found in normal human sera and seems to be released from activated dendritic cells and B-lymphocytes (Hock et al. (2001) Int. Immunol. 13: 959-967).
WO 97/29781 relates to methods and compositions (vaccines) for stimulating a humoral immune response in which a soluble form of CD83 is employed as an adjuvant together with a given antigen. Soluble forms comprise CD83 fusion protein and a soluble form consisting of amino acids 1 to 124, the extracellular domain of CD83. In addition to the use of CD83 as adjuvant for vaccine preparations, this document discusses the use of antagonists (antibodies) against CD83 for inhibiting undesirable antigen specific responses in mammals.
WO 93/21318 describes a CD83 protein here designated HB15, chimeric HB15 molecules and HB15 fragments including a fragment consisting of the extracellular domain (amino acids 1 to 125) of HB15. Furthermore antibodies against HB15 usable as pharmaceuticals are revealed. Because of the role of HB15 as an accessory molecule for lymphocyte activation, the soluble HB15 and fragments is thought to be useful as an agonist for augmentation of the immune response.
U.S. Pat. No. 5,710,262 and the corresponding WO 95/29236 reveal human and mouse HB15 as potentially useful drug in the treatment of AIDS (with regard to the DNA and amino acid sequence of mous HB15, see SEQ ID Nos:3 and 4). The extracellular domain of HB15 as described therein comprises the first 19 amino acids of the signal peptide, followed by 106 amino acids of the extracellular domain.
The above-mentioned WO 93/21318 and WO 95/29236 also emphasize that monoclonal antibodies against CD83 are suitable for removing endogenous CD83 or monitor CD83 levels in serum.
It was surprisingly found that the extracellular domain of CD83 (hereinafter also “hCD83ext”) comprising amino acids 20 to 144 (SEQ ID NO:2), can engage in heterophilic interactions with ligands on dendritic cells. Since the current literature only describes complete extracellular domains or extracellular domains lacking amino acids from the C-terminus of the extracellular domain (U.S. Pat. No. 5,710,262, WO 95/29236 and WO 97/29781) it was also surprising that hCD83ext adopted the correct confirmation, allowing interactions with dendritic cells. Of even greater surprise was the effect hCD83ext had on dendritic cells; it prevented maturation of immature dendritic cells and reduced the expression of CD83 in mature dendritic cells. As a result dendritic cells lost their ability to activate T cells. Thus, the soluble hCD83ext itself was shown to be suitable for the treatment or prevention of diseases or medical conditions caused by undesirable immune responses, in particular by preventing activation of T cells. hCD83ext was also found suitable for the treatment or prevention of diseases or medical conditions caused by undesirable immune responses mediated by dendritic cells and T cells.
It was found that due to the fact that the hDC83ext possesses the correct conformation of natural CD83, it is also suitable or preparing antibodies against CD83 (see Lechmann et al., Protein Expression and Purification 24, 445-452 (Mar. 5, 2002)).
Finally, it was found that the amount of soluble CD83 protein in the human serum varies and is significantly higher in case of tumors and B-cell leukemia. Thus, antibodies against the soluble CD83 protein are powerful tools for determining certain diseases (such as tumor, autoimmune diseases, viral infection, etc.) in a patient.